1. Field of the invention
The present invention-relates to the detection of ground faults, and, more particularly, to the detection of ground faults in electric heaters used to melt and thus remove snow and ice from pavement, roofs, gutters, down spouts, satellite dishes and the like.
2. Description of the related art
Electric heaters may be utilized to supply heat used in snow and ice melting systems. Typical melting applications include but are not limited to satellite dishes, roofs and gutters, pavement, building and garage entrances and facilities accommodating the physically challenged. Efficient operation requires embedding the electric heaters in or attaching the electric heaters to satellite dishes, pavement and other structures which may sometimes become covered with snow and ice.
Heater cable construction may employ one of several methods. For example, self-limiting heaters typically consist of two parallel stranded copper bus wires separated by a semiconducting polymer enclosed in one or more concentric layers of organic insulating material. Other common heater cable construction methods involve extruding a thermoplastic insulating compound over a single conductor or a pair of parallel conductors. Another construction method, the oldest, involves packing mineral insulation, commonly magnesium oxide, over a single conductor or a pair of parallel conductors enclosed within a copper or stainless steel tube. Current practice as dictated by the U.S. National Electric Code requires covering the heating cable with a grounded conductive copper braid or shield that serves as a return path for any ground current. Mineral insulated heaters accomplish this requirement by way of their outer stainless steel or copper tubular jackets.
Ground current is the difference between the outbound and return heater currents. The U.S. National Electric Code requires using a ground fault circuit interrupter (GFCI) on all snow and ice melting circuits. The GFCI interrupts heater current if the ground current exceeds a predetermined limit; usually 30 milliamperes. The GFCI requires manual reset after tripping. This preserves safety by not restarting heater operation during intermittent ground leakage current that may occur in wet locations.
Independent of the heater fabrication method, ground current can flow due to a heater failure caused by a manufacturing defect, corrosion, wear and tear or mechanical damage. Excessive ground current causes the dual safety problems of fire and shock hazard.
The fire hazard is variously referred to as a wet fire or heater burn-back. Although this can occur with heaters of any construction, it is more likely to occur in heaters with parallel conductors in the presence of moisture. Conductors exposed to the ambient due to mechanical damage are the starting point for the fire hazard. Moisture acting as an electrolyte on the cable in the area of the damage forms a conductive path between parallel conductors or between a conductor and a surrounding shield. Current flows through a small area and strikes an arc which creates a high temperature plasma. This carbonizes a portion of the polymer insulation and creates a conductive carbon arc track in the polymer. Flames and high temperatures occurring during the burn-back can ignite combustible materials in proximity to the heating cable. The burn-back mechanism in mineral insulated cable is similar except that magnesium hydroxide forms by mixing moisture with the magnesium oxide insulation to form a conductive electrolyte.
Aside from the fire hazard described above, an electrical shock hazard can also occur whenever ground current flows since its path to earth ground is usually not predictable. Thus, a GFCI is required to be incorporated into snow and ice melting electrical circuits.
Snow and ice melting systems commonly employ automatic controls that operate heaters only while required to minimize energy consumption and operating costs. Typically, the automatic controls sense ambient moisture and temperature. Heaters operate at ambient temperatures below a threshold--usually 38.degree. F. while ambient moisture is present and for a period of time thereafter to clear accumulated snow and ice. Optionally, the automatic control may inhibit heater operation at temperatures too low for effective melting, e.g., below 17.degree. F.
Current practice is to use a GFCI circuit breaker external to the automatic control of the snow and ice melting system. Such a self-contained GFCI circuit does not provide an output signal indicative of a ground fault condition. The automatic control may or may not require an external contactor for controlling heater operation.
It is also known from copending U.S. patent application Ser. No. 08/696,562, now U.S. Pat. No. 5,710,408, assigned to the Assignee of the present invention, to sense a ground fault condition by inductively measuring both the current flowing into the heating element and the current flowing out of the heating element. Any difference between these two current levels represents ground leakage current. If the ground leakage current exceeds a preset value, then a ground current interface sends a signal to a microcontroller. The microcontroller may shut down power to or otherwise control the heater based upon this signal.